A number of devices utilize small orifices to meter or control the flow of fluids therethrough. For example, some turbine engine parts are made from a material which includes a plurality of small holes therethrough and through which gases flow for transpirational cooling of the parts. Moreover, some fuel injectors for supplying precise quantities of fuel to internal combustions engines use an orifice plate for atomizing fuel injected into the combustion chamber of the engine. Such plates typically comprise a thin (0.004 in.) stainless steel disc having a number of small, diverging orifices extending through the disc. One such injector has six orifices (i.e., 0.006 in. diameter) angling through the disc at an angle of about 30.degree. to the principal plane of the disc so as to provide a fuel flow of about 2 g/sec in a cone-shaped pattern. This flow should not vary more than about one percent from one injector to the next.
Fuel injector orifices are currently made using electric discharge machining (EDM) techniques in which an electrical spark between a fine wire and the orifice plate selectively vaporizes metal from the disc to form the orifice. EDM machines are quite costly and typically machine only one orifice at a time. Moreover, in order to obtain a desired fuel flow rate through the orifice it is necessary to control a number of process variables other than flow rate to insure the requisite one percent variability allowance. In this regard, fuel flow rate through the disc is affected by a variety of geometrical parameters of the orifices in the orifice plate (i.e., diameter, length, taper, roundness, surface finish and presence of burrs). These geometrical parameters are, in turn, affected by the EDM process parameters (i.e., voltage, current, frequency, electrode gap, ramming speed, electrode type, physical metallurgy of the workpiece and the dielectric fluid's properties). With all these variables to consider it is extremely difficult to control the EDM process to keep it within the narrow window of allowable flow rate variation.
Electrolytic jet drilling has been used to drill metal substrates by passing electrolyzing current through an uncontained stream of electrolyte impinging on the surface of the metal to be drilled. While electrolyte splashes off and spreads over much of the surface of the substrate, drilling occurs substantially exclusively at the impingement site as is well know in the art. Such electrolytic drilling processes are known to make burr-free surfaces, but are incapable of accurately machining fuel injector sized orifices within a diameter tolerance of only 0.5 percent (i.e., one percent tolerance on flow equals a one percent tolerance on area and hence a 0.5 percent tolerance on orifice diameter).
It would be desirable if a process could be devised which would consistently and simply drill small, precisely sized fluid metering orifices with minimum fluid flow rate variability from one orifice to the next. It would even be more desirable if a process could be devised for simultaneously drilling a plurality of such orifices.
It is a principal object of the present invention to provide a process for the consistent electrolytic drilling of small orifices with little flow rate variability from one orifice to the next which process includes real time measurement of the flow rate through the orifice during drilling. It is a further object of the present invention to provide such a process for simultaneously electrolytically drilling a plurality of such orifices. These and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows and which is given hereafter in conjunction with several drawings in which: